Method and apparatus for producing a false color image

ABSTRACT

A method for creating a false color image, by utilizing a video camera provided with two or three arrays of semiconductor detectors, the sensitivity range whereof, in relation to wavelength, extends from the visible light range to near infrared radiation range, and where the radiation received in the semiconductor detectors is filtered. The radiation received in the first array of semiconductor detectors is filtered so that only near infrared radiation is given access to the detector array. Further, the radiation received in the second semiconductor detector array, or alternatively in the second and third semiconductor detector arrays is filtered so that of the radiation of the visible spectrum range, green and red are given access to the detector array or arrays. The signals obtained from the first and second semiconductor detector arrays, or alternatively from the first, second and third semiconductor detector arrays, representing the strength of near infrared radiation and the strength of the radiation of the green and red spectrum ranges, are decoded by changing the order of the signals in order to achieve a desired false color signal in the camera output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the methods for producing false color images.The invention also relates to apparatuses applying the said methods forproducing false color images.

2. Description of the Related Art

Semiconductor video cameras include a number of semiconductor detectors,generally silicon-based detectors, which are arranged in line form or inmatrix form. In addition, these detectors are generally coupled asCCD-detectors. Such CCD-detectors are sensitive for both visible lightand near infrared radiation, as is illustrated in FIG. 1. In color videocameras based on semiconductors, there are generally two or three arraysof CCD-detectors, the first array whereof is used for detecting bluecolor B, and the second for detecting combined green G and red R, oralternatively only green color G, in which case red color R is detectedby means of the third array. The spectrum ranges of different colorswith respect to wavelength are illustrated in FIG. 1.

In ordinary video cameras, the access of near infrared radiation to thedetectors is prevented by placing a filter cutting off near infraredradiation in front of the said detectors. The colors of the drawing,i.e. blue, green and red, are separated by means of another set offilters provided in front of the different detector arrays.

From the Finnish patent publication FI 79,641, there is known a methodand apparatus for producing a false color image. In the inventiondescribed in the said patent publication, the false color image iscreated so that from two channels, i.e. detector arrays, the effect ofnear infrared radiation was electrically reduced, whereafter the signalsR, G, B representing various colors were coded, and a video imageresembling a false color image was created.

A drawback with the known method and apparatus is, that the video signalis electrically processed for several times, which weakens the imagequality. Another drawback of the invention is that this kind ofelectrical arrangement for producing a false color image uses arelatively high amount of electric power, which is an obviousdisadvantage for instance while surveying the surroundings with a falsecolor camera from an aeroplane.

SUMMARY OF THE INVENTION

The object of the invention is to introduce a new method for producingfalse color images, which method makes use of semiconductor camerasknown as such. By means of the method and apparatus of the invention;the drawbacks of the known false color imaging method can be avoided,and the scanning can be realized in a remarkably simpler fashion thanbefore. This is achieved by the characteristic features of theinvention.

In the first method of the invention, there is utilized a video cameraprovided with two arrays of semiconductor detectors, the sensitivityrange whereof in relation to wavelength extends from the range ofvisible light to the near infrared range, and where the radiationobtained in the semiconductor detectors is filtered. According to theinvention, the radiation obtained in the first semiconductor detectorarray is filtered so, that only near infrared radiation is given accessto the detector array; the radiation obtained in the secondsemiconductor detector array is filtered so, that among the radiation ofthe visible spectrum range, green and red light are given access to thedetector array; and the signals obtained from the first and secondsemiconductor detector arrays and representing the strength of nearinfrared radiation and the strength of the radiation of the green andred spectrum ranges, are decoded by changing the order of the signals inorder to achieve the desired combined false color signal in the cameraoutput.

In another method of the invention, there is utilized a video cameraprovided with three arrays of semiconductor detectors with a sensitivityrange in relation to wavelength extends from the range of visible lightto the near infrared range, and where the radiation obtained in thesemiconductor detectors is filtered. According to the invention, theradiation obtained in the first array of semiconductor detectors isfiltered so, that only near infrared radiation is given access to thedetector array; the radiation obtained in the second array ofsemiconductors is filtered so, that of the radiation of the visiblespectrum range, green light is given access to the detector array; andthe radiation obtained in the third array of semiconductors is filteredso, that of the radiation of the visible spectrum range, red light isgiven access to the detector array; and the signals obtained from thefirst, second and third semiconductor detector arrays and representingthe strength of near infrared radiation, and the radiation of the greenand red spectrum ranges, are decoded by changing the order of thesignals in order to create a desired combined false color signal in thecamera output.

In a preferred embodiment of the method, the signals representing thestrength of the near infrared, green and red spectrum range radiations,are decoded by arranging them in the order green, red and infraredsignal in the video camera output.

In another preferred embodiment of the method, the signals representingthe strength of the near infrared radiation are decoded by arrangingthem in the reversed order green and infrared signal, whereas the signalrepresenting the radiation of the red spectrum range remains as such inthe video camera output.

An advantage of the invention is that a false color video image isproduced by means of optical filtering inside the video camera. Thus thesolution is simple, and the image quality is maintained good. Anotheradvantage of the invention is that the video camera of the inventiondoes not require additional electrical components. Thus the powerconsumption of the camera remains on the same level as that of anordinary video camera.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below, with reference to theappended drawings, where FIG. 1 illustrates the sensitivity range of aCCD-detector generally used in semiconductor video cameras; FIG. 2 is aschematic illustration of an ordinary video camera; and FIG. 3 is aschematic illustration of an apparatus of the invention for producing alive false color image.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The ordinary video camera 1 of FIG. 2 comprises three detector arrays 2,3 and 4, an optical system 5 as well as the infrared radiation filter 6and color separation filters 7, 8 and 9. The infrared radiation filter 6prevents the near infrared radiation contained in the incoming radiationfrom entering the detector arrays via the optical system 5. The colorseparation filters 7, 8 and 9 in turn are used for separating the bluespectrum range onto the first detector 2, the second color separationfilter 8 is used for separating the green spectrum range onto the seconddetector array 3, and the third color separation filter 9 is used forseparating the red spectrum range onto the third detector array 4. Thedetector arrays 2, 3 and 4 are connected in the output channels 10, 11and 12 of the camera, through which channels there are obtained thecorresponding color signals B, G, R, i.e. signals representing the blue,green and red spectrum ranges.

The detector arrays 2, 3 and 4 of an ordinary video camera can also begrouped so that the detector arrays 3, 4 together form a combineddetector array 13 (marked with dotted lines in FIG. 2). Respectively, infront of this detector array 13, there is provided a filter 14 (dottedlines again), which is permeable to the radiation of the green and redspectrum ranges. The green G and red R color signal are separated fromeach other at later stages by using known technique.

The apparatus of the invention for creating a false color image isillustrated in FIG. 3. The apparatus comprises a video camera providedwith three arrays of semiconductor detectors 2, 3 and 4, in similarfashion as in the ordinary video camera of FIG. 2. The apparatus alsoincluded an optical system 5 and color separation filters 7, 8 and 9,which are respectively arranged in connection with the detector arrays2, 3 and 4. By means of the filter 7, radiation of the green and redspectrum ranges, belonging to the visible wavelength range, is preventedfrom proceeding to the first detector array 2. By means of the filter 8,the radiation of the blue and red spectrum ranges is prevented fromproceeding to the second detector array 3. By means of the filter 9, theradiation of the blue and green spectrum ranges is prevented fromproceeding to the third detector array 4. However, near infraredradiation ir has free access through all filters 7, 8 and 9. By means ofthe optical system 5, the radiation entering the apparatus, i.e. theimage, is divided into three sub-images which are directed to thedetector arrays 2, 3 and 4.

In front of each detector array 2, 3 and 4 of the apparatus, there arearranged extra filters 15, 16 and 17 respectively. The first filter 15in front of the first detector array 2 attenuates the wavelengths of theblue spectrum range b. In front of the second and third detector arrays2, 3, there are arranged respective filters 16, 17 for attenuating nearinfrared radiation ir. From the first semiconductor detector array 2,there is thus received a signal representing the proportion of nearinfrared radiation ir in the video image; from the second semiconductordetector array 3 there is received a signal representing the proportionof the green spectrum range g in the video image; and from the thirdsemiconductor detector array 4, there is received a signal representingthe proportion of the red spectrum range r in the video image. At thisstage the said signals ir, g, r are in the above described order inrelation to the output channels 10, 11 and 12 or B, G and Rrespectively, of an ordinary video camera. Now, however, these signalsare fed into the a decoder 18, whereby the order of the signals in theoutput channels 10, 11, 12 of the camera is changed. To the input poles19¹, 19², 19³ of the decoder 18, there are respectively fed the signalsir, g and r. The input poles 19¹, 19², 19³ of the decoder 18 are coupledto output poles 20¹, 20², 20³ so that the input pole 19¹ is coupled tothe output pole 20¹ and the input poles 19² and 19³ to the output poles20² and 20³ respectively. Thus the signals in the output poles 10, 11and 12 of the camera are in respective order g, r and ir, i.e. theycorrespond to the signal outputs B, G and R of an ordinary video camera.

In the apparatus of FIG. 3, two detector arrays can be employed, i.e.the first detector array 2 and another detector array 13 combined of thedetector arrays 3 and 4, in similar fashion as in the video camera ofFIG. 2. Further, instead of the color separation filters 8, 9, there isadvantageously used a combined color separation filter 21. Incorresponding fashion, in front of the combined detector array 13, therealso is provided a combined extra filter 22. This extra filter 22 is anear infrared radiation filter, and corresponds to the single extrafilter 16 or 17. The combined signal g+r, received from the combineddetector array 13, representing the combined signal of green and redspectrum ranges, is fed into the decoder 18 via the combined input pole19², 19³, so that it is respectively obtained from the decoder via thecombined output pole 20¹, 20² to the combined output channel 10, 11 ofthe video camera, to be further processed in normal fashion.Consequently, from the combined output channel 10, 11 there is receivedthe signal g+r, which corresponds to the output signal B+G obtained froman ordinary video camera.

The order of the signals in the output of an apparatus of the inventioncan also be changed, by using the decoder 18, in another fashion thanthe one described above. In that case the input poles 19¹, 19² arecoupled to the output poles 20², 20¹ respectively, whereas the inputpole 19³ is respectively coupled to the output pole 20³. This isillustrated with dotted lines in FIG. 3. Now the signals in the outputpoles 10, 11 and 12 of the camera are in respective order g, ir, r, i.e.they correspond to the outputs B, G, R of an ordinary video camera. Analternative type of false color information is produced by means of thisarrangement.

In the above specification, the invention is described mainly withreference to one preferred embodiment, but it is clear that theinvention can be modified in many ways within the scope of theinventional idea defined in the appended patent claims.

We claim:
 1. A method for producing a false color image, using a videocamera provided with first and second arrays of semiconductor detectors,the first and second arrays having a sensitivity range in relation towavelength extending from visible light to near infrared radiation; andwhere radiation received in the first and second arrays is filtered, themethod comprising:filtering the radiation received in the first array toprovide that only the near infrared radiation (ir) is given access tothe first array; filtering the radiation received in the second array toprovide that only the radiation of green (g) and red (r) spectrum rangesare given access to the second array; and decoding signals (ir, g+r)received from the first and second arrays representing strength of thenear infrared radiation (ir) and strength of the radiation of the green(g) and red (r) spectrum ranges, by changing order of the signals toachieve a desired combined false color signal (g+r, ir) in a cameraoutput.
 2. A method for producing a false color image, utilizing a videocamera provided with first, second, and third arrays of semiconductordetectors, the first, second, and third arrays having a sensitivityrange in relation to wavelength extending from visible light to nearinfrared radiation; and where radiation received in the first, second,and third arrays is filtered, the method including the stepsof:filtering the radiation received in the first array to provide thatonly the near infrared radiation (ir) is given access to the firstarray; filtering the radiation received in the second array to providethat only the radiation of green (g) spectrum range is given access tothe second array; filtering the radiation received in the third array toprovide that only the radiation of red (r) spectrum range is givenaccess to the third array; decoding signals (ir, g, r) received from thefirst, second and third arrays representing strength of the nearinfrared radiation (ir) and strength of the radiation of the green (g)and red (r) spectrum ranges, by changing order of the signals to achievea desired combined false color signal (g, r, ir) in a camera output. 3.The method of claim 2 for producing a false color image, wherein thedecoding step includes rearranging the signals representing the strengthof the near infrared radiation and the radiation from the green (g) andred (r) spectrum ranges into an order green (g), red (r) and infrared(ir) in the camera output.
 4. The method of claim 2 for producing afalse color image, wherein the decoding step includes rearranging thesignals representing the strength of radiation of the green (g) spectrumrange and the near infrared radiation (ir) in reversed order into green(g) and near infrared (ir), whereas the signal representing theradiation of the red (r) spectrum range is maintained in the cameraoutput.
 5. An apparatus for producing a false color image, comprising:avideo camera provided with first and second arrays of semiconductordetectors and a first color separation filter system, the first andsecond arrays having a sensitivity range in relation to wavelengthextending from visible light to near infrared radiation; a decoder; anda second filter system including,a first filter for attenuatingwavelengths of blue light (b), the first filter being arranged incooperation with the first array of semiconductor detectors to providethat received radiation is filtered so that only the near infraredradiation (ir) is given access to the first array; a second filter forattenuating the near infrared radiation (ir), the second filter beingarranged in cooperation with the second array of semiconductor detectorsto provide that received radiation is filtered so that only theradiation of red (r) and green (g) spectrum ranges are given access tothe second array; and that signals (ir, g+r) obtained from the first andsecond arrays and representing strength of the near infrared radiation(ir) and strength of the radiation of the green (g) and red (r) spectrumranges, are decoded in the decoder by changing an order of the signalsto achieve a desired combined false color signal (g+r, ir) in an outputof the camera.
 6. An apparatus for creating a false color image,comprising:a video camera and a first color separation filter system,the video camera including first, second, and third arrays ofsemiconductor detectors; a decoder; and a second filter systemincluding,a first filter attenuating wavelengths of blue light (b), thefirst filter being arranged to cooperate with the first array to providethat received radiation is filtered so that only near infrared radiation(ir) is given access to the first array; a second filter attenuating thenear infrared radiation (ir), the second filter being arranged tocooperate with the second array to provide that received radiation isfiltered so that only the radiation of green (g) spectrum range is givenaccess to the second array; a third filter (17) attenuating the nearinfrared radiation (ir), the third filter being arranged to cooperatewith the third array to provide that received radiation is filtered sothat only the radiation of red (r) spectrum range is given access to thethird array; wherein signals (ir, g, r) obtained from the first, secondand third arrays and representing strength of the near infraredradiation (ir) and strength of the green (g) and red (r) radiation, aredecoded in the decoder by changing an order of the signals to achieve adesired combined false color signal (g, r, ir; g, ir, r) in an output ofthe camera.